In electronic control devices such as an invertor, servo amplifier, and spindle amplifier, a semiconductor device for power control comprises large capacity diodes, transistors, IGBTs, and MOSFETs. There is a so-called "DBC board" as a board for a circuit used in a electronic circuit generating a large quantity of heat such as a power control section. The DBC board comprises a metal plate for a conductor circuit, a ceramic insulating material, and a heat-conductive metal plate joined to a base of said ceramic insulating material. The metal plate used in the DBC board, for instance may be a copper plate. The ceramic insulating material used, for instance, may be alumina ceramic or alumina ceramic nitride. The heat-conductive metal plate, for instance, may be a copper plate. If such a material as alumina ceramic or nitriding alumina ceramic nitride, each with a high heat conductivity, is used as a ceramic insulating material, heat generated in a conductor circuit is rapidly transferred to a metal plate. Furthermore, as the material itself is ceramic, degradation of the material due to corona discharge is suppressed, and also the resistance against electric discharge is improved.
In the DBC board, however, as a coefficient of thermal expansion of the ceramic insulating material layer is different from that of the heat conductive metal plate layer, the heat cycle characteristic is rather poor, and cracks may be generated in the joining interface. Also, due to restrictions in the manufacturing process of ceramics, it is difficult to manufacture a large size plate. Typically, a ceramic product is limited to a dimension of around 150 mm.times.150 mm, using the conventional manufacturing method. In addition, the heating of materials at a high temperature is required in a process of manufacturing the conventional board, and as a result, the manufacturing process is very complicated with a high production cost.
To solve the problems with the DBC board as described above, use of a metal base board in which an insulating layer made of organic polymer and metallic foil, and adhered to a heat-conductive metal plate with an adhesive, has been proposed. As shown in FIG. 7, in this metal base plate 70, a circuit conductor 3 is formed via an insulating layer 2 comprising an organic insulating material on the upper surface of the heat-conductive metal plate 1, in which a granular inorganic filler material is added to an insulating layer 2. This configuration is disclosed, for instance, in the Japanese Patent Publication No. 6235/1971 and the Japanese Patent Publication No. 9650/1972. A discharge path 71 is seen between conductor 3 and plate 1.
Generally, organic polymer constituting an insulating layer has a substantially lower heat conductivity as compared to that of ceramics used in the DBC board, but the heat conductivity is raised by adding such an inorganic filler material as alumina or silica. Furthermore, the heat conductivity is raised to a desired level by reducing the thickness of the insulating layer to around 7% to 30% of that of the DBC board.
The technological documents relating to this invention includes the Japanese utility Model Laid Open Publication No. 106775/1991 disclosing a "metal base board", the Japanese Utility Model Laid open Publication No. 73966/1988 disclosing an "heat-radiating insulating board", the Japanese Utility Model Laid Open Publication No. 98253/1987 disclosing a "printed-circuit board", the Japanese Patent Laid Open Publication No. 244180/1991 disclosing a "Metal base laminated plate", the Japanese Patent Laid Open Publication No. 27786/1990 disclosing a "low heat resistance circuit board", the Japanese Patent Laid Open Publication No. 232795/1989 disclosing a "Method of Manufacturing a Metal Base Board", and the Japanese Patent Laid Open Publication No. 232792/1990 disclosing a "Circuit Board".
Specifically, FIG. 1 or FIG. 4 in the Japanese utility Model Laid Open Publication No. 106775/1991 disclose a metal base board, in which lengthy inorganic fillers having a high heat conductivity are added in an organic insulating layer. The longitudinal surfaces thereof are facing the metal base side or are disposed at random. Because the inorganic fillers are added, heat radiated from parts mounted to the circuit conductor 3 of a metal base board 70 such as transistors, resistors, and capacitors, can effectively be transferred for emission to the outside.
Furthermore, as an organic polymeric insulating material is used in these metal base boards, a board which is larger than a DBC board can easily be manufactured with a low cost. Furthermore, there is no problem with the formation of cracks due to a heat cycle, as in a DBC board. For the reasons as described above, a metal base board having the configuration as described above is employed in a power circuit or other similar electric components which generate a large quantity of heat.
In the conventional type of metal base board as described above, however, a granular inorganic filler material is added to an organic polymeric material and the resultant mixture is used as an insulating material. However, the organic insulating layer is easily degraded when exposed to electric discharge, and the granular filler material does not work effectively. Namely, when degradation due to electric discharge proceeds up to the section with the granular inorganic filler material added therein, the degradation will continue to proceed along the section interface. As a result, the filler is not always effective in improving the electric discharge resistance characteristics.
In addition, as the thickness of the insulating layer is reduced to approximately 7% to 30% of the thickness of an ordinary DBC board to insure a desirable heat radiating characteristics, the insulating characteristics will be affected, under high voltage. First, it should be noted that, if the thickness of an insulating layer is increased, the resistance against electric discharge is improved, but the heat radiating characteristics becomes lower because heat generated in the circuit conductor is hardly transferred to the metal plate. Also, it should be noted that, although a conventional type of metal base board is generally used in applications requiring a relatively low operating voltage, i.e., AC 200 V or below, recently a metal base board is often used in applications where a high voltage in the class of 400 V or 600 V is always applied. Thus, under these two conditions, the insulating layer made of organic polymer will be degraded due to corona electric discharge, and the insulation, characteristics of the insulating layer will be damaged. In addition, as the thickness of a plate is small, the life until the final breakage of insulation will be short.
There are several reasons for this problem. First, if there is air on a surface along a conductor circuit section of a board, and even if the section has been processed with, for instance, resist or a silicon potting material, there exist void sections with air filled therein in the processed layer. Second, it is known that, if a high AC voltage is applied to the such air-filled void sections, corona electric discharge is generated. Third, in most organic polymeric materials a voltage at which corona electric discharge starts is in a range of approximately 400 to 500 V, although it depends on the thickness of each board. Not only in electronic equipment having a rated voltage of 400 V or less, but also in electronic equipment for which a rated voltage is more than 400 V, if a voltage of more than 400 V is loaded during its service operation in association with electronic control of such sections as a switch, corona electric discharge may occur.
Furthermore, although the "metal base board" disclosed in the Japanese Utility Model Laid Open Publication No. 106775/1991 is effective to provide thermal advantages, such as improvement of heat conductivity and reduction of coefficient of linear thermal expansion, degradation due to electric discharge will proceed along an interface, as previously described for the board shown in FIG. 7 with a granular inorganic filler material added therein when a degradation due to electric discharge reaches the fillers. For this reason, the filler can not improve a resistance against electric discharge of a metal base board.